1. Field of the Invention
The present invention relates to an oxymethylene polymer resin composition and an extrusion molded article thereof. More particularly, the present invention is concerned with an oxymethylene polymer resin composition comprising an oxymethylene polymer, a sterically hindered phenol compound, a low-density polyethylene, a calcium salt of a fatty acid having 10 to 36 carbon atoms, wherein the calcium salt has very low contents of calcium ions and chlorine ions which are derived from impurities, and a substance containing a formaldehyde-reactive nitrogen atom. The invention is also concerned with an extrusion molded article produced from the resin composition. The resin composition of the present invention is advantageous not only in that it has excellent extrusion moldability and exhibits excellent anti-slip properties when contacting the screw of an extruder during extrusion molding, but also in that an extrusion molded article produced therefrom has a remarkably reduced occurrence of whitened portions and void portions comprising microvoids and hence has excellent mechanical properties and an excellent appearance.
2. Prior Art
Oxymethylene polymers have been known as engineering plastics which have a good balance of mechanical properties and excellent moldability, and have been used in the production of a wide variety of articles, such as automobile parts, parts for electric and electronic equipment, industrial sundry goods, toys and the like. Among these articles, small-size and mass-produced articles are mainly made by an injection molding method. On the other hand, large-size articles and non-mass-produced articles are usually made by a method in which an oxymethylene polymer is subjected to extrusion molding, and the resultant extrusion molded article is subjected to secondary processing, such as cutting. Representative examples of articles produced by extrusion molding include a cylindrical bar having a diameter of from about 10 to 200 mm (hereinafter referred to as a "rod"), a sheet having a thickness of from about 10 to 100 mm (hereinafter referred to as a "large-thickness sheet"), and the like.
The inherent moldability of oxymethylene polymers is unsatisfactory for producing molded articles, such as a rod and a large-thickness sheet, by extrusion molding. Specifically, the obtained extrusion molded articles are disadvantageous in that they have an interior portion which exhibits a color tone different from that of the surrounding portions (such interior portion is hereinafter referred to as a "whitened portion") or they have an interior void portion comprising microvoids and the like. The occurrence of these whitened and void portions unfavorably leads to a lowering of not only mechanical properties but also the appearance of the molded articles. For example, when a screw is produced by cutting a rod (disadvantageously containing a void portion) which is produced from an oxymethylene polymer by extrusion molding, the mechanical strengths (such as flexural strength, tensile strength and impact strength) of the screw are reduced at the void portion therein, so that the screw is more likely to break under stress. Thus, it has been desired to reduce the occurrence of a whitened portion and a void portion in extrusion molded articles produced from oxymethylene polymers.
In order to improve the moldability of oxymethylene polymers, various attempts have been made. However, many of these attempts are intended to solve problems which accompany the injection molding of an oxymethylene polymer. Examples of these attempts include methods for improving mold release properties [see Unexamined Japanese Patent Application Laid-Open Specification No. 59-4641, Unexamined Japanese Patent Application Laid-Open Specification No. 60-104151, Unexamined Japanese Patent Application Laid-Open Specification No. 63-295661 (corresponding to U.S. Pat. No. 4,900,769) and Unexamined Japanese Patent Application Laid-Open Specification No. 4-100847 (corresponding to U.S. Pat. No. 5,187,218)]; methods for improving warpage resistance (see Unexamined Japanese Patent Application Laid-Open Specification No. 51-97652, Unexamined Japanese Patent Application Laid-Open Specification No. 55-16049, Unexamined Japanese Patent Application Laid-Open Specification No. 57-128740, Unexamined Japanese Patent Application Laid-Open Specification No. 60-104152, Unexamined Japanese Patent Application Laid-Open Specification No. 62-91551 and Unexamined Japanese Patent Application Laid-Open Specification No. 2-50948); and methods for improving shrinkage resistance (see Unexamined Japanese Patent Application Laid-Open Specification No. 4-100818 and Unexamined Japanese Patent Application Laid-Open Specification No. 4-100848). However, by these methods, the occurrence of whitened portions and void portions in extrusion molded articles cannot be satisfactorily prevented.
On the other hand, attempts have been made to obtain molded articles having various desired mechanical properties by molding a mixture of an oxymethylene polymer and a polyolefin. Examples of these attempts include a method for obtaining a fibrous article having improved stretching properties, in which use is made of a molding material obtained by adding a polyethylene to an oxymethylene polymer (see Examined Japanese Patent Application Publication No. 41-2730); a method in which use is made of a molding material obtained by adding a polyolefin resin to an oxymethylene polymer so as to increase the strength of the ultimate molded article (see Examined Japanese Patent Application Publication No. 42-19498); a method in which a composition comprising an oxymethylene polymer and a polybutene is used as a molding material (see Examined Japanese Patent Application Publication No. 43-20378), a method in which a composition comprising an oxymethylene polymer, an .alpha.-olefin polymer and a copolymer of ethylene and a vinyl monomer is used as a molding material (see Unexamined Japanese Patent Application Laid-Open Specification No. 49-40346); and a method in which a composition comprising a polyacetal, a polyethylene and a lubricant is used as a molding material [see Unexamined Japanese Patent Application Laid-Open Specification No. 49-104941 (corresponding to U.S. Pat. No. 4,051,096)]. Further, several attempts have been made in which use is made of a molding material obtained by adding a polyolefin, a sterically hindered phenol compound and the like to an oxymethylene polymer. For example, Unexamined Japanese Patent Application Laid-Open Specification No. 50-52166 discloses a Working Example in which use is made of a molding material obtained by mixing a polyoxymethylene diacetate with a fine powder of a low-density polyethylene, 2,2'-methylene-bis-(4-methyl-6-t-butylphenol), a copolyamide and isoeicosanol acetate; and Unexamined Japanese Patent Application Laid-Open Specification No. 50-103556 discloses a Working Example in which use is made of a molding material obtained by mixing a polyoxymethylene diacetate with a low-density polyethylene, 2,2'-methylene-bis-(4-methyl-6-t-butylphenol), a copolymer nylon and isostearyl alcohol, and polyethylene oxide. Unexamined Japanese Patent Application Laid-Open Specification No. 49-058145 (corresponding to U.S. Pat. No. 3,980,734) discloses the use of a thermoplastic molding material in which a polymer having a specific freezing temperature and a specific molecular weight is dispersed in a particulate form having a particle size of from 0.1 to 5 .mu.m in a mixture of a linear polyoxymethylene and a network-structure polyoxymethylene, wherein the dispersed polymer is selected from an .alpha.-olefin homopolymer and an .alpha.-olefin copolymer. Unexamined Japanese Patent Application Laid-Open Specification No. 2-166151 discloses a composition comprising a polyoxymethylene, a sterically hindered phenol compound having a molecular weight of 300 or more, at least one substance selected from the group consisting of an alkali metal hydroxide, an alkaline earth metal hydroxide, an alkali metal salt of a carboxylic acid and an alkaline earth metal salt of a carboxylic acid, and a saponification product of a copolymer of ethylene and vinyl acetate. Further, Unexamined Japanese Patent Application Laid-Open Specification No. 6-207080 discloses a polyacetal resin molding material obtained by a method in which a polyolefin and additives (specifically a heat stabilizer, a heat aging resistance improver and a weathering resistance improver) are added to a polyacetal resin, and the resultant polyacetal resin composition is subjected to crystallization for a specific period of time. By these prior art techniques, an oxymethylene polymer can be improved with respect to abrasion resistance, impact resistance, stiffness, heat stability, dimensional stability and the like. However, in these prior art documents, there is no description suggesting that the prior art techniques can suppress the occurrence of whitened portions and void portions in extrusion molded articles. Further, these prior art techniques cannot satisfactorily prevent the occurrence of whitened portions and void portions in extrusion molded articles.
For preventing the occurrence of whitened portions and void portions in extrusion molded articles, Unexamined Japanese Patent Application Laid-Open Specification No. 6-212054 (corresponding to U.S. Pat. No. 5,519,075) proposes a composition comprising an oxymethylene polymer, a sterically hindered phenol compound, an olefin polymer resin, a polyalkylene glycol, a specific amide compound and melamine. This composition is improved in that the occurrence of whitened portions and void portions can be suppressed to some extent; however, the suppression is unsatisfactory. Further, due to the presence of a polyalkylene glycol in this composition, the composition is likely to slip on the screw of an extruder during extrusion molding, so that the extrusion molding of the composition cannot be efficiently performed, thus leading to a lowering of the productivity of desired extrusion molded articles.
Unexamined Japanese Patent Application Laid-Open Specification No. 7-324155 discloses a polyacetal resin composition containing a calcium salt of a fatty acid, wherein the calcium salt has a specific calcium ion content and a specific chlorine ion content. This composition is described to be improved in heat aging resistance, heat stability and mold deposit prevention. However, this prior art document has no description about an improvement in suppression of the occurrence of a whitened portion and a void portion in extrusion molded articles.